feat: scaffold backend and shared library

src/ASTRAIN.Api/Services/UserKeyGenerator.cs

@@ -0,0 +1,22 @@
+using System.Security.Cryptography;
+using System.Text;
+
+namespace ASTRAIN.Api.Services;
+
+public static class UserKeyGenerator
+{
+    private const string Alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789";
+
+    public static string Generate(int length = 8)
+    {
+        var buffer = new byte[length];
+        RandomNumberGenerator.Fill(buffer);
+        var chars = new char[length];
+        for (var i = 0; i < length; i++)
+        {
+            chars[i] = Alphabet[buffer[i] % Alphabet.Length];
+        }
+
+        return new string(chars);
+    }
+}